Revert of Implement overlay scrollbar fade out for non-composited scrollers.

third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm

Index: third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm
diff --git a/third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm b/third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm
index a49351808fc115c54467c0f1efafa3d14ffc4027..79356732195909885ce20fbac12ebf74221af731 100644
--- a/third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm
+++ b/third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm
@@ -952,6 +952,20 @@
   [m_scrollbarPainterController.get() setHorizontalScrollerImp:nil];
 }
 
+bool ScrollAnimatorMac::shouldScrollbarParticipateInHitTesting(
+    Scrollbar& scrollbar) {
+  // Non-overlay scrollbars should always participate in hit testing.
+  if (ScrollbarThemeMac::recommendedScrollerStyle() != NSScrollerStyleOverlay)
+    return true;
+
+  // Overlay scrollbars should participate in hit testing whenever they are at
+  // all visible.
+  ScrollbarPainter painter = scrollbarPainterForScrollbar(scrollbar);
+  if (!painter)
+    return false;
+  return [painter knobAlpha] > 0;
+}
+
 void ScrollAnimatorMac::notifyContentAreaScrolled(const ScrollOffset& delta) {
   // This function is called when a page is going into the page cache, but the
   // page